In an image forming apparatus such as an electrophotographic apparatus or the like, an electrostatic latent image is formed, and then the electrostatic latent image is developed by toner or the like, and a developed toner image is fixed to a printing sheet. In order to fix the image to the printing sheet, it is necessary to apply heat to the toner image and, normally, the fixation is achieved by heating the printing sheet having the toner image held thereon from the front surface with a fixing roller and applying a pressure from the back surface with a press roller.
For example, heating of the fixing roller is achieved by winding a fixing belt between the fixing roller and a heat roller, heating the heat roller by a heater integrated in the heat roller, transferring the heat to the fixing belt, and transferring the transferred heat to the fixing roller (for example, JP-A-2001-318557).
The printing sheet to be subjected to the fixation of the toner image includes those in various sizes, and these printing sheets have different widths. For example, when the fixation is carried out on a printing sheet having a small width, the temperature of a portion of the fixing roller which actually carries out the fixation falls. In contrast, end portions which do not carry out the fixation are kept at a raised temperature, and hence might be overheated.
In this manner, when printing an image using a small size sheet, the temperature of the portion of the fixing roller where the printing sheet passes falls because the printing sheet absorbs the heat therefrom, so that the temperature control is achieved by heating. In contrast, it is known that the portion of the fixing roller where the printing sheet does not pass is overheated because the heat is not absorbed by the sheet, so that the temperature rise is resulted.
In order to prevent the temperature rise as such, JP-A-2001-318557 discloses an image forming apparatus in which a heat source (heat lamp) for a large size sheet is turns ON for the fixation of a large size sheet, and a heat source for a small size sheet and the heat source for the large size sheet controlled to a low power are used for the fixation of the small size sheet. In this publication, there are descriptions such that the heat source for the large size sheet may be provided within the press roller instead of within the heat roller (see FIG. 9) and the fixing belt is heated from the outside by a cleaning roller (having the heat source for the large size integrated therein) which comes into contact with the fixing belt (see FIG. 6).
When heating the press roller as well, since the portion which is free from contact of the printing sheet might be overheated when the fixation of the printing sheet having a small width is carried out, it is also contemplated to prevent the temperature rise by providing a heat source for the large size sheet and a heat source for the small size sheet in the press roller.
When the heat roller or the fixing belt (heat belt) is heated by the heat source (heater) provided in the heat roller or the cleaning roller as described above, a large heat capacity is required, and hence it takes a significantly long time from the power-on of the device until the temperature of the portion of the fixing belt which comes into contact with the press roller rises to a predetermined temperature. Therefore, there is a problem such that copying cannot be started immediately after the power-on of the image forming apparatus.
In order to shorten the rising time after the power-on, a system to heat the fixing belt directly from the outside by an induction heating (IH) or the like is contemplated. According to the fixing belt direct heating system, since the heat capacity is small, the time required for heating the surface of the fixing belt to a predetermined temperature is shortened, so that the rising time after the power-on is shortened correspondingly. In contrast, since the temperature fall tends to occur in the portion of the fixing belt where the printing sheet comes into contact with, it is necessary to heat this portion, while the temperature rise tends to occur in the portion where the printing sheet does not come into contact with. Therefore, it is contemplated to use a separate induction heat device obtained by dividing the induction heat device which heats the fixing belt directly from the outside into a center portion and both side portions.
The invention relates to a fixing device having a separate induction heat unit which divides the fixing belt into sections in the widthwise direction and induction-heats the same and a multiple-heater integrated press roller having a plurality of integrated heaters to be used according to the width of the printing sheet.
Referring now to the drawings, problems of the fixing device having the structure as described above will be described.
According to the induction heat device having a divided structure, the fixing belt can be heated by selectively driving the plurality of heaters having a heating portion at positions different axially from each other according to the sheet width. However, the sheet width does not necessarily match the positions where a coil is divided, and the printing sheet might pass over part of the width of a divided induction coil. In such a case, the temperature fall occurs in the portion of the fixing belt where the printing sheet passes, while the temperature fall does not occur where the printing sheet does not pass, and hence partial overheating or temperature fall cannot be avoided if the belt is uniformly heated.
FIG. 1 shows a state where the printing sheet passes over part of divided induction heat coils as described above. It is assumed that an induction heat coil 13 includes a center coil 13a, and side coils 13b and 13c provided on both sides thereof with respect to a fixing belt 12, and temperature sensors 15a, 15b, and 15c sense passage of the printing sheet with respect to the center coil 13a and the side coils 13b and 13c. In this case, as shown in the drawing, when a printing sheet P1 having a width W1 passes, that is, the printing sheet passes so that the side edges thereof match the divided positions of the divided coils, the temperature sensor 15a senses the passage of the printing sheet, and the temperature sensors 15b and 15c do not sense the passage of the printing sheet. Therefore, only the center coil 13a is energized, and hence only the center portion of the fixing belt is heated, so that an adequate temperature control is achieved. However, when a printing sheet P2 having a width W2 passes the induction heat device, the passage of the printing sheet is sensed or not sensed depending on the positions where the temperature sensors 15b and 15c are installed. For example, it is assumed that the temperature sensor 15b is provided at the center of the side coil 13b and the temperature sensor 15c is provided on the outer end of the side coil 13c as shown in FIG. 1. In this case, since the temperature sensor 15c is provided at the end, the temperature fall is not sensed even though the printing sheet P2 passes and the side coil 13c is not heated. In contrast, since the temperature sensor 15b is provided at the substantially center of the side coil 13b, the temperature sensor 15b senses the temperature fall due to the printing sheet P2 and the passage of the printing sheet is sensed, so that the side coil 13b is heated. However, since the printing sheet does not pass a portion outside the center of the side coil 13b, the temperature does not fall, and hence the overheated state is resulted.
In particular, when heating directly from the outside of the fixing belt by an electromagnetic induction heating, the fixing belt (heat belt) as a heating object have a low heat capacity in many cases. Therefore, if the heating by the respective divided side coils continues, the temperature of the non-sheet-passing portion of the fixing belt might rise. Also, if an attempt is made to restrain the temperature rise of the non-sheet-passing portion by lowering the temperature of the side end portion of the fixing belt to be induction heated, the temperature rise in the non-sheet-passing both end portions is restrained, but the temperature of the fixing belt in the inner end of the side coils (the end portions of the side coils 13b and 13c near the center coil 13a in FIG. 1) is also lowered. Therefore, the temperature of the portions of the fixing belt in the widthwise direction corresponding to the side coils 13b and 13c where the printing sheet passes falls, there is a problem such that the defective fixation in these portions or the defective image quality such that the gloss of the printed image is fluctuated might occur in a portion corresponding to the center coil and portions of the side coils near the center coil.
The invention provides a fixing device which solves the problems of the fixing device of a separate induction heat type using a fixing belt or a fixing roller in the related art.